plotting two sets of contours

[ahw199(at)soton.ac.uk (Ann Webber)]

I am trying to plot two different sets of contours over one image. The
contour sets are both centered around different points, So I need to
position them separately. I can overplot one set at the right point
but when it comes to the second set the position keyword doesnt seem
to work and the second set are overplotted over the first set.
Do I need to combine all the data into one file, i.e. work out the two
plots in relation to each other or can I use some other keyword to
position the second set?
A sample of my programme follows:
WINDOW, XSIZE=830, YSIZE=800
ERASE,255
READ_JPEG,'S:\Ann webber\drawing3white.jpg',a,image
TV, a,140,205
TEK_COLOR
OPENR,1,'my filename'
A=FLTARR(9,145000)
READF,1,A
close,1
CONTOUR,A(7,*),A(0,*),A(1,*),/irregular,levels=[6,8,10,15,18,20,25,30,40,50,60,70,80,90,100,150,200],color=2,C_LABELS=[1,1,1,1,1,1,1,1,1,0,1,0,1,0,1,0,1]
$
,position=[515,495,645,625],/device,/noerase,/overplot
OPENR,2,'my filename'
B=FLTARR(9,40401)
READF,2,B
close, 2
;CONTOUR,(B(7,*)/1.414),B(1,*),B(0,*),/irregular,levels=[10,15,20,25,30,40,50,60,70,80,90,100,150,200],color=4,C_LABELS=[1,1,1,1,1,1,1,1,1,0,1,0,1,0,1,0,1]
$
;,position=[400,420,515,535],/overplot